The invention relates to a tensioning unit for a strapping machine for the tensioning and sealing of a band guided around an item which is to be strapped. The tensioning unit has a tensioning head and a housing accommodating the same. The tensioning head includes a tensioning device and a clamping and sealing subassembly. Toward the end of a respective tensioning operation, the tensioning head can be displaced relative to the housing in a band-running direction, counter to the tensioning direction. Provided in the housing is a band guide which is arranged downstream of the clamping and sealing member, as seen in the band-running direction.
By means of such a tensioning unit, the band is usually first of all introduced into a curved guide path element of the strapping machine and thus guided around the respective item which is to be strapped. The band end is then clamped firmly by means of the clamping and sealing member. Before the band is tensioned by the tensioning device, band guides provided are moved back to release the band. The band is then tensioned by the tensioning device. Friction forces occur at the different corners of the item around which the band is guided. A decrease in the band tensioning, is caused on account of this friction, from corner to corner of the item which is to be strapped. Thus, toward the end of the tensioning operation, the tensioning head is displaced, relative to the housing accommodating said tensioning head, in a band-running direction, counter to the tensioning direction, in order to ensure that the tensioning is regulated correspondingly. In this case, the item which is to be strapped is secured in relation to the housing, for which purpose the housing may be connected, for example, firmly to the curved guide path element of the strapping machine.
In the case of conventional tensioning units, it is not just the band guide which is arranged downstream of the clamping and sealing member, as seen in the band-running direction. The entire rear band guide is correspondingly displaced in the band-running direction, with the tensioning head, relative to the housing.
Since, the entire rear band guide is correspondingly displaced and has to leave enough space in order to be able to pass out of the housing, the housing has to be provided with a corresponding through-passage. For this purpose, corresponding intervention in the curved band-guide path element, which adjoins the housing, is also necessary.
An object of the invention is to provide a tensioning unit in which it is possible to dispense with the through-passages in the housing and which can thus be installed without any intervention in the adjacent components being necessary.
This object is achieved according to the invention in that the band guide, which is arranged downstream of the clamping and sealing member, is subdivided into a front and a rear band-guide part. Only the front band-guide part can be displaced, together with the tensioning head, relative to the housing. The rear band-guide part can be moved away generally transversely to the band-running direction by the front band-guide part, when the latter is moved in the band-running direction, preferably counter to a spring force.
With a respective displacement of the tensioning head, the rear band-guide part is thus displaced away within the housing, generally transversely to the band-running direction, by the front band-guide part. As a result, enough space is provided within the housing for the movement of the front band-guide part. The rear band-guide part does not move along with the tensioning head in the band-running direction. Thus, there is no longer any need for any through-passages in the housing. The tensioning unit can be installed in the relevant strapping machine without any intervention in the adjacent components. This is advantageous, in particular, for installation in existing machines.
The front and the rear band-guide parts may be provided with interacting sloping surfaces via which the rear band-guide part is correspondingly displaced away. If the tensioning head, and the front band-guide part, are displaced in the band-running direction, then the rear band-guide part slides, by way of its sloping surface, along the sloping surface of the front band-guide part. For example, in the outward direction, as a result of which a corresponding amount of space is provided for the front band-guide part.
In one exemplary embodiment of the tensioning unit according to the invention, the rear band-guide part is mounted such that it can be pivoted about an inner spindle, supported on the housing. The rear-band guide part is forced into its band-guiding position by a spring force. If the band is to be released for the tensioning operation, then the rear band-guide part can be correspondingly deflected counter to the spring force. If, toward the end of the tensioning operation, the tensioning head is then displaced in the band-running direction, the rear band-guide part can additionally be deflected counter to the spring force in order to provide enough space for the front band-guide part. The relevant pivot spindle preferably extends generally parallel to the band-running direction.
The rear band-guide part, which is preferably arranged at a free end of a pivot lever, may be provided with a band-guide slit. As soon as said band-guide part is pivoted out of its band-guiding position, the band can pass out of the guide slit.
In one exemplary embodiment of the tensioning unit according to the invention, the front band-guide part can be driven for a respective release of the band. The rear band-guide part is correspondingly carried along by the front band-guide part. The rear band-guide part thus already has the front band-guide part correspondingly acting on it for a respective band release.
It is possible for the front band-guide part to be mounted such that it can be pivoted. For example, about a spindle, and to be forced into its band-guiding position by a spring force. Once again, it is expedient for the pivot spindle generally to extend parallel to the band-running direction.
The front band-guide, part, may be preferably arranged at a free end of a pivot lever with a band-guide slit. The latter is aligned with the guide slit of the rear band-guide part. If the rear band-guide part is carried along, for example, by the front band-guide part for a respective release of the band, and is correspondingly pivoted with said front band-guide part, then the band is released simultaneously from both slits.
In one exemplary embodiment of the tensioning unit according to the invention, the front band-guide part, which is provided with a band-guide slit, is arranged at a free end of a two-armed pivot lever. At the other free end is provided a follow-on roller interacting with a cam plate. In this case, it is possible for the pivot lever to be forced into the band-guiding position by a spring force.
It is customary for the tensioning head also to comprise a band guide which is arranged upstream of the clamping and sealing member, as seen in the band-running direction. Such a front band guide may then be moved out of its band-guiding position again for a respective release of the band.
The invention is explained in more detail hereinbelow, by way of an exemplary embodiment, with reference to the drawings.